Cyclic imines are a group of secondary metabolites produced by marine dinoflagellates that are characterised by spiro-linked cyclic imines, macrocycles and ether rings (Cembella and Krock (2008); Roach et al (2009); Selwood et al (2010); Van Wagoner et al (2011)). Compound families included in this group are spirolides (Hu et al (1995)), pinnatoxins (Uemura et al (1995)), gymnodimines (Seki et al (1995)), pteriatoxins (Takada et al (2001)), prorocentrolides (Torigoe et al (1988)), spiro-prorocentrimines (Lu et al (2001)) and symbioimines (Kita et al (2005)). In all these compounds, the cyclic imine moiety consists of a six- or seven-membered ring with a spiro-link to a cyclohexene ring.
Pinnatoxin A was first described in 1995 from an Okinawan bivalve (Uemura et al (1995)) and a range of congeners of the compound Pinnatoxin A have since been reported. However, it is only recently that a pinnatoxin-producing benthic microalga, Vulcanodinium rugosum, was reported from New Zealand waters (Rhodes et al (2011); Nézan and Chomérat (2010)). Strains of this dinoflagellate have now been found in many locations around the world, including Australia, Japan, China, France and Hawaii.
The toxin profile of V. rugosum varies among strains, producing a single pinnatoxin, a mixture of pinnatoxins or no pinnatoxins (Rhodes et al (2011); Nézan and Chomérat (2010)). Shellfish accumulate pinnatoxins and can metabolise them to other analogues including fatty acid esters (McCarron et al (2012)). Administration of pinnatoxins and other cyclic imines to mice leads to death by respiratory failure (Munday (2008)).
Cyclic imines have been shown to block muscular and neuronal nicotinic acetylcholine receptors (Kita and Uemura (2005); Hellyer et al (2011)), and the observed death by asphyxia is consistent with inhibition of neuromuscular transmission, leading to paralysis of the diaphragm. To date there have been no reports of human intoxication by shellfish contaminated with cyclic imines (Kita and Uemura (2005)).
Secondary metabolites produced by marine organisms are recognised as a source of lead compounds for drug development (Molinski et al (2009)). Analyses of the property distributions of drugs, natural products and compounds from combinatorial chemistry have shown a better correlation between those for drugs and natural products, than between those for drugs and compounds from combinatorial chemistry (Feher and Schmidt (2003)).
A target for drug development by rational design is the regulation of apoptosis, otherwise known as programmed cell death. Apoptosis is a cell suicide mechanism by which multicellular organisms remove damaged or unwanted cells in order to maintain normal life development and homeostasis (Fuchs and Steller (2011)).
Members of the BCL-2 family of proteins are functionally classified as either antiapoptotic or proapoptotic. The regulation of the interaction of these proteins dictates survival or commitment to apoptosis. Central to the initiation of apoptosis is the activation at the outer mitochondrial membrane of the proapoptotic BCL-2 family members BAK and BAX. Upon activation these proteins homo-oligomerize into proteolipid pores to promote mitochondrial outer membrane permeabilization. Following mitochondrial outer membrane permeabilization, caspase activation and apoptosis ensue. The caspase proteases effectively package and label dying cells for rapid clearance (Chipuk et al (2010)).
The BCL-2 family of proteins is implicated in a range of diseases of which cancer is the best characterised. A prevalent mechanism of apoptosis dysregulation in cancer is overexpression of antiapoptotic family members, e.g. BCL-2 or BCL-xl in lymphoma and epithelial cancers. Inhibiting the action of these proteins has therefore been used as a target for the development of drugs for use on the treatment of subjects with neoplasia. The compound designated ABT-737 and its orally bioavailable counterpart designated ABT-263 are inhibitors of the antiapoptotic action of BCL-2 family members. The compounds ABT-737 and ABT-263 have been shown to be effective as a synergist or a single agent, respectively, in selectively killing tumour cells (van Delft et al (2006); Oltersdorf et al (2005)).
It is an object of the present invention to provide a standard for use in monitoring the occurrence in the environment or harvested marine organisms of the compound I. It is an object of the present invention to provide lead compounds for drug development. These objects are to be read disjunctively with the object to provide at least a useful choice.